1. Field of the Invention
The present invention relates to an apparatus and process for removing strong oxidizing agents from liquids and, more particularly, to an apparatus and process for substantially reducing chlorine concentration in water.
2. Description of the Related Art
High purity water is required in many manufacturing and analytical applications in, for example, the chemical industry, the foodstuffs industry, the electronics industry, medical industry, and the pharmaceutical industry. These applications typically require treatment of a source water supply (such as a municipal drinking water supply) to reduce the levels of contaminants. Treatment processes and systems typically include combinations of: distillation, filtration, ion exchange (including electrodeionization), reverse osmosis (RO), photo oxidation, ozonation, and ultrafiltration.
In an effort to decrease biological contaminants in municipal water supplies, especially in warmer environments, the addition of chlorine has become commonplace. As used herein, the term "chlorine" can include various chlorine containing compounds, such as chloramines, chlorine dioxide, chlorite, chlorate, perchlorate, and the like. While effective as a biocide, chlorine becomes a contaminant itself in certain applications which are environmentally sensitive or require high purity water. Furthermore, it is widely known in the art that strong oxidizing agents, such as chlorine, have a deleterious effect on certain thin film membranes, such as those used in RO units, as well as on ion exchange resins. In this regard, municipal water supplies typically contain chlorine concentrations (e.g., approximately 1 ppm) that are unacceptable for these sensitive applications. Other strong oxidizing agents often found in municipal water supplies include percarbonate, perborate, peracetate, bromine, iodine, peroxide, and ozone. Any of these agents can be a contaminant in a given application.
In the medical industry, for example, purified water is used in various applications, such as for dialysis. Water for dialysis is typically produced by anion exchange using, for example, an electrodeionization unit to remove ionized contaminants from the water. As noted, strong oxidizing agents, such as chlorine, are known to have a deleterious effect on anion exchange resins. Chlorine, in particular, can react with anion exchange resins to form nitrosamines, a group of carcinogenic compounds which are known to cause cancer in a number of organs, including the liver and kidneys.
Nuclear and fossil-fuel power plants also have stringent water quality requirements to reduce corrosion and scaling and the associated expensive downtimes. In pressurized water reactor nuclear plants, for example, high-purity water is important in reducing corrosion in steam generators. In boiling water reactor nuclear plants, high-purity water is important in maintaining water quality in the nuclear reactor. Traditionally, makeup water treatment systems for power plants have relied almost exclusively on various combinations of filtration, ion-exchange, and reverse osmosis, with the latter two being especially sensitive to strong oxidizing agents.
Similarly, In the pharmaceutical industry, various degrees of purified water are used in drug manufacture, injection of drugs, irrigation, and inhalation. The United States Pharmacopoeia (USP) lists standards for the various types of water used in the pharmaceutical industry, including purified water, sterile purified water, water for injection, sterile water for injection, sterile bacteriostatic water for injection, sterile water for irrigation, and sterile water for inhalation. The prescribed source water for pharmaceutical grade waters is "drinking water" as defined by Environmental Protection Agency regulations.
Purified water can be used to process certain drugs, particularly as a cleaning agent for equipment and in the preparation of certain bulk pharmaceuticals. Purified water is produced from drinking water by pretreatment equipment followed by ion exchange, and/or RO, and/or distillation.
Sterile purified water is not used in any drug that will be introduced directly into the bloodstream. Purified water is made sterile by heating it to a minimum temperature of 121.degree. C. for at least 15 minutes. Water for injection can be introduced directly into a patient's bloodstream and, therefore, must meet all purified water standards and endotoxin limits. In the processing of water for injection, a RO or distillation unit must be used.
Sterile water for injection can be used to dilute drugs which will be introduced into the bloodstream. Sterile water for injection is packaged in volumes not larger than 1 liter and is made sterile by the process noted above. Sterile bacteriostatic water for injection is similar to sterile water for injection including antimicrobial agents, and is packaged in volumes not larger than 30 milliliters. Sterile water for irrigation is used during surgical procedures to flush tissue within the body. Sterile water for inhalation is similar to sterile water for injection that is used in inhalers and in the preparation of inhalation solutions.
FIG. 1 illustrates a schematic process flow diagram of a typical prior art water treatment system 10 for producing purified water for various applications. As shown, feedwater, typically municipal drinking water, is fed through line 12 to a multimedia filter unit 14 to remove bulk particulate material. The water is then passed through a water softener 16, most typically an ion exchange unit, to remove scale-forming cations such as calcium and magnesium. In addition, water softener 16 serves to remove divalent and trivalent cations and reduce the tendency for coagulation of colloids that could foul downstream RO membranes. The water is then passed through a heat exchanger 18, which is typically used in system 10 if the source water is from a surface water source such as a lake or river.
The water is passed from heat exchanger 18, if used, to a dechlorination unit 20 that can include an activated carbon bed to reduce the chlorine concentration, which, as noted above, is typically present in the municipal drinking water that serves as the source water for the system. After dechlorination, the water is passed to a cartridge filtration unit 22, which provides a final filtration to protect the RO membranes from fouling or other damage caused by relatively large particles generated from upstream equipment. The water is then passed to an RO unit 24, which removes nearly all of the particulate material from the feedwater. Typically, greater than 98 percent of dissolved substances are rejected by the RO membrane. Although not shown, a double-pass configuration of RO units can be used to achieve high quality purified water.
Optionally, the permeate from the RO unit(s) can be passed to a distillation unit 26 for the production of water for injection. A storage tank 28 may also be provided to store the distilled water prior to its use in production and/or packaging in unit 30. In alternative applications, such as used in the power industry or the electronics industry for example, Other water treatment units may be substituted for distillation unit 26. Suitable substitutions may include, for example, ion exchange units and the like.
As noted above, municipal drinking water is often chlorinated to control pathogenic microorganisms. Two treatment methods that are commonly used for chlorine removal are granulated activated carbon (GAC), and the direct injection of reducing agents such as sodium dioxide, sodium sulfite, sodium bisulfite, sodium metabisulfite, or other SO.sup.2 - bearing chemicals. Other strong, scale-forming oxidizing agents can be removed by injection of various compounds. The type and concentration of the compounds necessarily depend on the type and concentration of oxidizing agent present and can include ascorbic acid, hydrazine, carbohydrazide, morpholine, and the like.
For RO pretreatment, dechlorination units containing a GAC bed are known to effectively reduce aqueous chlorine concentrations by the following reactions:
C+HOCl.fwdarw.CO+H.sup.+ +Cl.sup.-,
and EQU C+OCl.sup.-.fwdarw.CO+Cl.sup.-
Unfortunately, however, chlorine reacts with GAC to form carbon monoxide and carbon dioxide, thereby depleting the media. GAC also tends to adsorb microorganisms and other contaminants on its surface. The adsorbed contaminants in a GAC bed eventually occlude catalytic sites, thereby reducing the bed's useful life. When this occurs, the exhausted media must be replaced and may require special handling. Additionally, the surface of GAC provides a favorable site for subsequent microbial growth, which may be unacceptable in biologically sensitive applications.
The direct injection of reducing agents into a process stream for removal of strong oxidizing agents is typically limited to applications that do not require a potable product. Although effective, this method has many disadvantages, including chemical cost, handling, storage, and the inconvenience and cost associated with metering and monitoring equipment. Sodium metabisulfite, in particular, can break down to form sodium sulfate, which stimulates sulfate reducing bacteria. Furthermore, the bulk removal of strong oxidizing agents by oxidation/reduction (redox) reactions introduces unwanted ionic and other species into the process stream, which may contribute to scaling of RO membranes.
Other, more recent, developments for chlorine removal include the use of flow-through reaction vessels containing metal redox media. However, because of the spontaneous redox reactions that take place in the reaction vessel, this dechlorination method can provide some of the same problems associated with sulfite addition. Specifically, when chlorine concentrations are moderate to high (i.e., approaching 1 ppm or more), the metal redox media can introduce unacceptably high levels of metal ions into the process stream, resulting in problems for downstream equipment.
Ultraviolet (UV) irradiation has been successfully used to reduce the levels of active microorganisms and organic compounds in water treatment systems. In particular, irradiation with UV light in the 185-254 nm wavelength range has been shown to be an effective germicidal (bacteriocidal and bacteriostatic) treatment for water. The adsorption of UV light by the DNA and proteins in the microbial cell results in the inactivation of the microorganism. A combination of lamps producing UV radiation in both the 185 nm and 254 nm ranges has also been shown to be effective in photooxidating organic compounds.
Recently, preliminary work has focused on UV dechlorination, which would have several inherent advantages over conventional dechlorination methods. Besides energy, UV radiation does not add anything to the treatment stream, such as undesirable chemicals and attendant color, odor, taste, and the like, nor does it generate harmful byproducts. UV irradiation would also provide beneficial side effects such as disinfection, total organic carbon (TOC) reduction, and ozone (O.sub.3) destruction. One such study entitled "Ultraviolet Dechlorination of Beverage Water," published in The Society of Soft Drink Technologists, pp. 71-94, (1988) and carried out by Coca-Cola USA, was directed to UV dechlorination of trace amounts of residual chlorine downstream of a GAC filter. The results show that uneconomically high UV dosage requirements clearly favor treatment of trace levels of free chlorine and chloramines rather than use of this technique as a primary means of dechlorination. In this regard, the raw data showed that a residence time of 10 to 50 times that necessary for disinfection was necessary to remove trace amounts of chlorine (i.e., 0.10 to 0.24 ppm). Even more telling are the results of a "real world" test that more accurately emulated the irradiation unit's effect on raw municipal drinking water (i.e., [Cl.sub.2 ]=approximately 1 ppm). From an economic standpoint, the flowrates needed for the essentially complete chlorine removal necessary for RO pretreatment were unsatisfactory (i.e., less than 3.43.times.10.sup.-2 gpm).
The results of another study were published in the April, 1998 Edition of ULTRAPURE WATER in an article entitled "CHLORINE REMOVAL--ULTRAVIOLET LIGHT OXIDATION OF CHLORINE IN WATER." This study was directed to the feasibility of using commercially available UV units for dechlorinating process water. Based in part on results showing that UV could economically remove trace amounts of chlorine, the authors concluded that free chlorine levels around 1.0 ppm may be impractical and unrealistic for UV oxidation, and that GAC and chemical injection will continue to be the primary means of dechlorination.
As government regulations become more stringent and the demand for high purity water continues to grow, new and improved methods of treating municipal water to obtain high purity water are needed. To date, there is no efficacious and economically acceptable treatment method that removes moderate to high concentrations of strong oxidizing agents from municipal water supplies without introducing corresponding moderate to high concentrations of other contaminates into the treatment stream.